This invention relates to an improved method for hydrolyzing and condensing dimethyldichlorosilane so as to prepare a cyclic siloxane of the general formula: EQU [(CH.sub.3).sub.2 SiO]).sub.m I
where m is a whole number predominantly ranging from 3-6, inclusive. This preparation of cyclic polysiloxanes is accomplished by hydrolyzing dimethyldichlorosilane in the presence of the anionic surfactant, sodium lauryl sulfate (hereinafter identified as "sulfate") having the formula ##STR1##
By means of this invention, the dimethyldichlorosilane when hydrolyzed gives a number of cyclic polysiloxanes according to formula I which predominantly are of the type where m is 3-6 with a minimum of linear dimethylsiloxanes terminated with silanol groups.
Cyclic polysiloxanes, particularly cyclic dimethyl siloxanes, such as those in formula I where m is 4 (also called "tetramer") or m is 5 (also called "pentamer"), or even where m is 6 ( also called "hexamer") are extremely desirable in the silicone art. Even the lowest cyclic polysiloxane where m is 3 (also called "trimer") has a value, but is usually formed in smaller amounts than either the tetramer or the pentamer.
These cyclic dimethylsiloxanes have many uses. From these cyclic polysiloxanes there may be prepared silicone rubbers, silicone fluids, including silicone lubricating oils, etc. These cyclics can be used by themselves for foam suppression in certain applications.
It is commercially important that in preparing these polysiloxanes that they be in a cyclic form since, from a purity viewpoint when making silicone gums the cyclics are the starting materials used with alkali metal hydroxides as condensing agents. Thus, the silicone gums are generally prepared by heating the cyclic tetramer with KOH to form the gum which in turn is filled with reinforcing fillers, such as fume silica, and then molded with a curing agent, such as benzoyl peroxide, to form heat-resistant silicone products.
In making silicone fluids, it is generally desirable to react cyclic polysiloxane in the above category with hexamethyldisiloxane with a mineral acid so that linear polysiloxanes are formed with chain-stopped trimethylsiloxy units or any other organosiloxy units which it may be desired to insert into the polymer chain.